Electro-optical device having a ratio controlling means for providing gradated display levels

ABSTRACT

An electro-optical device comprising a display drive system with the display timing related to the unit time t for writing-in a picture element and to the time F for writing-in one picture is disclosed. In the device, a gradated display corresponding to the ratio of the division can be obtained by time-sharing the signal during a write-in of time t without changing the time F.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electro-optical device and moreparticularly to an active-type liquid crystal display device in whichclear gradated display levels can be set.

2. Description of the Prior Art

Because of the physical characteristics of the liquid crystalcomposition, the dielectric constant thereof differs between a directionparallel to the molecule axis and a direction perpendicular to themolecule axis, which is referred to as dielectric anisotropy, thereforethe liquid crystals can easily be arranged parallel to or perpendicularto an external electric field. A liquid crystal electro-optical deviceutilizes this dielectric anisotropy, so that the ON/OFF display isachieved controlling the amount of light transmitted or the amount oflight dispersion.

The electro-optical characteristics of a nematic liquid crystal areshown in FIG. 2. The relationship between the applied voltage and thetransmissivity (amount of light transmitted) is as follows;

    ______________________________________                                        the applied voltage     the light transmitted                                 ______________________________________                                        smaller  Va at point A 201        0%;                                                  Vb at point B 202                                                                            about     30%;                                                 Vc at point C 203                                                                            about     80%;                                        larger   Vd at point D 204                                                                            about    100%.                                        ______________________________________                                    

In short, if only points A and D are utilized, the two gradations, blackand white, are displayed, and if the rising portion of theelectro-optical characteristic curve is utilized, such as at points Band C, an intermediate gradated display is possible.

It was confirmed that Va=2.0 V, Vb=2.18 V, Vc=2.3 V and Vd=2.5 V.

Conventionally, in the case of a liquid crystal electro-optical devicewith a gradated display utilizing a TFT, the applied gate voltage orvoltage applied over the source and drains of the TFT is varied toadjust the voltage, so that an analogue gradated display is obtained.

The gradated display method with a liquid crystal electro-optical deviceutilizing TFTs is further described below in detail.

An n-channel thin film transistor conventionally utilized in a liquidcrystal electro-optical device has the voltage-current characteristic asshown in FIG. 20. In the drawing, numeral 301 designates thecharacteristic in case of an n-channel thin film transistor usingamorphous silicon, while numeral 302 designates the characteristic incase of an n-channel thin film transistor using polycrystalline silicon.

In a conventional gradated display method, by controlling analogvoltages to be applied to the gate electrode, drain currents can becontrolled and accordingly the resistance value between the source andthe drain can be changed. As a result, the strength of the electricfields to be applied to the liquid crystal connected thereto in seriescan be arbitrarily changed by the division of the resistance, whereby agradated display is made possible.

Also there is another method, where the gate electrode is connected toscanning signal lines and the voltage between the source and drain ischanged, resulting in controlling arbitrarily the electric field valueitself to be applied to the liquid crystal.

Both of the above methods are analogue gradated display methods largelyrelying upon the TFT characteristics. It is however difficult to formnumbers of TFTs for matrix composition so as to make all of them have anuniform electric characteristic. Particularly, it is extremely difficultin the present circumstances to finely adjust the intermediate voltagenecessary for a gradated display by the present techniques. As can berealized by the electro-optical characteristics of a nematic liquidcrystal shown in FIG. 2, a gradated display has to be carried out within0.32 V, that is, from around 2.08 V, the boundary value of darkcondition, to around 2.40 V, the boundary value of light condition. Inthe case of a gradated display of 16 gradations, the control of thevoltage at every 0.02 V in average is required.

On the other hand, when the voltage is controlled at such as point A 201and point D 204 shown in FIG. 2 where liquid crystal is completelyturned ON/OFF, the difference between voltages of 0.5 V or more can beobtained, which will sufficiently ease the variation in TFTcharacteristics. When, using a plurality of write-in frames, for example6 frames among 10 frames are turned ON (at 2.5 V) and the remaining 4frames are turned OFF (2.0 V), the write-in voltage is 2.3 V in average,so that an intermediate gradated display becomes possible.

In this case, however, the drive frequency might be decreased to 30 Hzor lower which is not discernible by the human eyes. Depending onconditions, this becomes a cause of the inferiority of a display such asflicker. Although it is proposed to raise the drive frequency to preventthe above problem, the data transfer speed of a driver IC has its limitup to about 20 MHz.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a means ofsupplying a precise, clear level of gradated display to the liquidcrystals by presenting a digital gradated display rather than aconventional analogue gradated display.

In order to obtain a good quality gradated picture, the drive frequencyof the drive IC is raised and the frame frequency is not reducedsubstantially and the frame frequency does not fall below the visuallydiscernable minimum frequency (the lowest confirmed number of frames) inthe method of displaying a gradated image in accordance with the presentinvention.

With the present invention, a gradated display is provided in anactive-matrix-type liquid crystal display device, using a display drivesystem with the display timing related to the unit time t for writing-ina picture element and to the time F for writing-in one picture, wherein,by time-sharing the signal during a write-in of time t, without changingthe time F, a gradated display corresponding to the ratio of thedivision is obtained since an average electric field applied to a liquidcrystal at a picture element (pixel) during the time t can be controlledby controlling the ratio of the division.

For purposes of explanation, the type of 4×4 matrix shown in FIG. 3 willbe used.

In the case of a method for gradated display with a conventional displaydevice, as shown in FIG. 4, the electric fields like 220-223 to beapplied to the pixel electrodes are determined depending on the strengthof the electric fields to the signal lines 210-213 of the data electrodedirection, from which the transmittance of the liquid crystals isdetermined. Note reference numerals in FIG. 3 corresponds to that inFIG. 4.

In the present invention, this type of analogue gradation control is notused, and the signal during a write-in unit time t 225 for writing in apixel is time-shared as shown in FIG. 1, so that the gradated displaycan be accomplished with each of the divisions used as a minimum unit.

At this time, in the case where the electric fields 227, 229, 231 in thewrite-in time are changed as shown in FIG. 1, the electric fields in thenon-write-in time become the average values 228, 230, 232, and a cleargradated display becomes possible.

For another explanation, the type of 2×2 matrix shown in FIG. 10 or FIG.17 will be used.

In another method for gradated display with a conventional displaydevice, as shown in FIG. 11, a plurality of picture frames, for example,16 frames are used, and the electric field over the picture elementelectrodes is determined as the average voltage for 16 frames by turningthe picture elements ON and OFF, from which the transmittance of theliquid crystals is determined.

In the present invention, however, the conventional analogue gradationcontrol or the gradated display with a plurality of frames is not used,and the signal during a write-in unit time t 325 for writing in a pixelis time-shared as shown in FIG. 9 and FIG. 16, so that the gradateddisplay can be accomplished with each of the divisions used as a minimumunit. In the case of the circuit shown in FIG. 10, a gradated displaycan be obtained with the signal in FIG. 9, and in the case of thecircuit shown in FIG. 17, a gradated display is obtained with the signalin FIG. 16.

At this time, in the case where the electric fields 327, 329 in thewrite-in time are changed as shown in FIG. 9 or FIG. 16, the liquidcrystals are activated by the average value of the applied voltage and aclear gradated display becomes possible.

In another method of the present invention, a digital gradated displayis carried out without changing the frame frequency and with the datatransfer frequency and the frequency for gradated display beingindependent of each other.

In the case of a liquid crystal electro-optical device having 1920×400dots, for example, the data transfer on the information signal side by 8bit parallel transfer requires the clock frequency of 5.76 MHz. If theconventional method with a plurality of frames, e.g. 10 frames, isemployed for this data transfer, the clock frequency as high as 57.6 MHzis necessary. However, since the clock frequency for a gradated displayis made independent in the present invention, a gradated display havingabout 166 gradations is possible with an IC driven at 8 MHz in maximum.If an IC driven at 12.3 MHz is used, a display having 256 gradations,which is considered necessary for a visual display, is sufficientlypossible. Therefore, the gradated display in accordance with the abovemethod is greatly advantageous over the conventional analogue gradateddisplay and the gradated display with a plurality of frames.

Liquid crystal components which can be utilized in the present inventionare a material exhibiting ferroelectricity, a material exhibitinganti-ferroelectricity, a material consisting mainly of a nematic liquiedcrystal, a material consisting mainly of a cholesteric liquid crystal, anematic liquid crystal dispersed in an organic resin, a cholestericliquid crystal dispersed in an organic resin, and a smectic liquidcrystal dispersed in an organic resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an example of the drive waveforms in thisinvention;

FIG. 2 is a graph showing the electro-optical characteristics of thenematic liquid crystal;

FIG. 3 is an electric circuit of the NTFT matrix;

FIG. 4 is a graph showing an example of the drive waveforms in the priorart device;

FIG. 5 is a partly cut-away plan view of the device of an embodiment inthe present invention;

FIGS. 6(A) to 6(G) are cross-sectional views to show the manufacturingprocess of an example of this invention;

FIG. 7 shows examples of picture elements in display in this invention;

FIG. 8 shows another example of the drive waveforms in this invention.

FIG. 9 is a graph showing an example of the drive waveforms inaccordance with the present invention;

FIG. 10 shows an example of the circuit configuration of a liquidcrystal electro-optical device in accordance with the present invention;

FIG. 11 is a graph showing another example of conventional drivewaveforms;

FIG. 12 shows a layout of electrodes and the like of one embodiment ofthe present invention;

FIGS. 13(A) to 13(F) are cross-sectional view to show the formationprocess of a substrate of the present invention;

FIG. 14 is a gradated display obtained in one embodiment of the presentinvention;

FIG. 15 is a graph showing another example of the drive waveforms in thepresent invention.

FIG. 16 is a graph showing another example of the drive waveform inaccordance with the present invention;

FIG. 17 is another example of the circuit configuration of a liquidcrystal electro-optical device of the present invention;

FIGS. 18(A) to 18(F) are cross-sectional views to show the formationprocess of a substrate of one embodiment of the present invention;

FIG. 19 is a graph showing an example of the drive waveforms in thepresent invention;

FIG. 20 shows gate voltage-drain current characteristics in apolycrystalline silicon TFT and an amorphous silicon TFT.

FIG. 21 is another example of the circuit configuration of a liquidcrystal electro-optical device of the present invention;

FIGS. 22(A) to 22(I) are cross-sectional views to show the manufacturingprocess of another example of the present invention;

FIGS. 23(A) to 23(E) are cross-sectional views to show the formationprocess of a substrate of the present invention;

FIG. 24 shows a schematical configuration of an electro-optical deviceof one embodiment of the present invention;

FIG. 25 shows the peripheral circuitry of a liquid crystalelectro-optical device of the present invention;

FIGS. 26(A) to 26(E) are cross-sectional views to show the manufacturingprocess of a liquid crystal device of one embodiment of the presentinvention;

FIG. 27(A-B) shows an assembly of a projection type image display of oneembodiment of the present invention;

FIGS. 28(A) to 28(G) are cross-sectional views to show the manufacturingprocess of a liquid crystal display device of one embodiment of thepresent invention;

FIG. 29 shows a schematic configuration of a liquid crystalelectro-optical device of one embodiment of the present invention;

FIG. 30 shows a schematic configuration of a reflection type liquidcrystal dispersion display device of one embodiment of the presentinvention;

FIG. 31 is an example of the circuit configuration of a liquid crystaldisplay device of the present invention;

FIG. 32 is a layout of electrodes and the like of another embodiment ofthe present invention;

FIGS. 33(A) to 33(H) are cross-sectional views to show the manufacturingprocess for a liquid crystal panel of one embodiment of the presentinvention;

FIG. 34 shows the peripheral circuitry of a liquid crystalelectro-optical device of the present invention;

FIGS. 35(A) to 35(D) are graphs showing input signal waveforms inputtedto and output signal waveforms outputted from the C/TFT obtained in oneembodiment of the present invention;

FIGS. 36(A) to 36(G) are cross-sectional views to show the manufacturingprocess for a liquid crystal display device of one embodiment of thepresent invention;

FIG. 37 is a layout of electrodes and the like of another embodiment ofthe present invention;

FIG. 38 is a layout of electrodes and the like of still anotherembodiment of the present invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

In this embodiment, a liquid crystal display device with the circuitconfiguration shown in FIG. 17, that is, a buffer type circuitconfiguration, is used.

FIG. 38 shows the layout of the actual electrodes and the likecorresponding to the circuit configuration of FIG. 17.

In order to simplify the explanation, the parts corresponding to a 2×2matrix only are described.

Also, the actual driving signal waveform is shown in FIG. 16. Forsimplicity, the explanation of the signal waveform is also given for thecase of 2×2 matrix configuration.

The manufacturing process for forming the substrate for the liquidcrystal display device used in an embodiment of the present invention isshown in FIG. 18.

In FIG. 18(A), a silicon oxide film for a blocking layer 951 having athickness of 1000 to 3000 angstroms was formed on a glass substrate 950,using a magnetron RF (high frequency) sputtering method. The glasssubstrate 950 was made of inexpensive glass capable of withstanding heattreatment up to 700° C. e.g. of 600° C.

The process conditions were as follows:

atmosphere: 100% oxygen

film forming temperature: 150° C.

output: 400 to 800 W

pressure: 0.5 Pa

The film forming, using either quartz or single-crystal silicon for atarget had a speed of 30 to 100 Angstroms/minute.

On top of this construction, an amorphous silicon film was formed usingan LPCVD (low pressure chemical vapor deposition) method, sputteringmethod, or a plasma CVD method.

If using the LPCVD method to form the silicon film, disilane (Si₂ H₆) ortrisilane (Si₃ H₈) was supplied to the CVD apparatus at a temperature100° to 200° C. less than the crystallization temperature, that is, 450°to 550° C., for example at 530° C. The pressure inside the reactionfurnace was 30 to 300 Pa. The film forming speed was 50 to 250 Å/minute.

In order to control the threshold voltage (Vth) of both the NTFT andPTFT at a substantially same level, boron with a concentration of 1×10¹⁵to 5×10¹⁸ cm⁻³ can be added using diborane.

In the case of using a sputtering method, the conditions were asfollows;

back pressure before sputtering: up to 1×10⁻⁵ Pa

target: a single crystal silicon

atmosphere: argon with hydrogen 20 to 80% by volume e.g. 20 volume % Arand 80 volume % H₂

the film forming temperature: 150° C.

frequency: 13.56 MHz

sputter output: 400 to 800 W

pressure: 0.5 Pa

In the case of forming a silicon film using the plasma CVD method, thetemperature was 300° C. for example, and monosilane (SiH₄) or disilane(Si₂ H₆) was used as the reactive gas, which were input into a PCVDapparatus and 13.56 MHz high frequency electric power was applied forfilm forming.

In this method, it is preferable that the oxygen concentration of thefilm formed is 5×10²¹ cm⁻³ or less. If the oxygen concentration ishigher than this range, crystallization becomes difficult and the heatannealing temperature must be high or the annealing time long. On theother hand, if the concentration is too low, the current leak in the OFFstate increases because of the back light. For this reason theconcentration is held in the range from 4×10¹⁹ to 4×10²¹ cm⁻³. Siliconconcentration was assumed to be 4×10²² cm⁻³ and hydrogen concentrationwas 4×10²⁰ cm⁻³, which is equal to one atomic % of the siliconconcentration.

Also, to promote further crystallization at the source and drain, theoxygen concentration is adjusted to 7×10¹⁹ cm⁻³ or less, or, preferably,1×10¹⁹ cm⁻³ or less, and oxygen may be added by ion implantation to aconcentration range of 5×10²⁰ to 5×10²¹ cm⁻³, only to the channelforming regions of the TFTs which form the pixels. On the other hand, itis effective for a high frequency operation to reduce the amount ofoxygen contained in the TFT provided in the peripheral circuits which nolight reaches, so as to make the carrier mobility greater.

After the amorphous silicon film was formed to a thickness of 500 to5000 Å, for example 1500 Å by means of the foregoing process, anintermediate heat treatment was performed at 450° to 700° C. for 12 to70 hours in an oxygen-free atmosphere, for example, in a hydrogenatmosphere at 600° C.

Because an amorphous silicon oxide film was formed on the surface of thesubstrate beneath the silicon film, no specific nucleus existed in thisheat treatment so the whole amorphous silicon film was uniformly heatannealed. Specifically, the amorphous structure was kept in film formingand hydrogen was merely mixed in.

It was supposed that when annealing the silicon film, crystallizationwas inclined to take place in a highly ordered state from the amorphousstructure, so that a crystal state was partly produced. Particularly, inthe regions where a relatively highly ordered state was produced justafter film forming of silicon, the tendency of crystallization to acrystal state was strong. A junction, however, took place due to thesilicon between these regions, so that the silicon attracted each other.

According to laser Raman spectrometry measurement of the annealedsilicon film, it was observed that a peak thereof was shifted from 522cm⁻¹ of the single crystal silicon to a lower frequency side. Theapparent grain diameter, when calculated using the half-value width, was50 to 500 Å, like a micro crystal. Actually, there were many of thesehighly crystallized regions that made up clusters. Each cluster wasjoined to the other by a silicon junction (anchoring) forming asemi-amorphous film.

Consequently, it was believed that the film could be said to havesubstantially no grain boundary (GB). The carriers could easily travelbetween the clusters through the anchored areas, so the mobility of thecarriers was higher than that of the polycrystalline silicon havingclear grain boundaries (GB).

The Hall mobility obtained was (μh)=10 to 200 cm² /Vsec, and theelectron mobility obtained was (μe)=15 to 300 cm² /Vsec.

When the film is polycrystallized at a higher temperature of 900° to1200° C. and not an intermediate temperature above, segregation ofimpurities occurs due to the growth of solid phase from nucleus in thefilm, and there are a lot of impurities such as oxygen, carbon, nitrogenin GB. Therefore, the mobility is large in the crystal, but the movementof carriers is prohibited by the barrier at GB. Consequently, it isactually impossible to obtain the mobility of 10 cm² /Vsec. or more.

This is a reason why a silicon semiconductor with a semiamorphous orsemicrystalline structure is used in this embodiment. Of course, othercrystalline semiconductor materials having a high mobility can be usedin the present invention.

FIG. 18(A) shows the silicon film which was photoetched using a firstphotomask ○91 , with the NTFT region 913 (20 μm in channel width)prepared at the left side of the drawing and the PTFT region 922 at theright side.

A silicon oxide film was then formed as a gate insulation film in thethickness range of 500 to 2000 Å, for example, 1000 Å. This is preparedunder the same conditions as the silicone oxide film formed as ablocking layer. A small amount of fluorine may be added to this film forfixation of the sodium ion during film forming.

When this operation was completed, a silicon film was providedcontaining a 1 to 5×10²¹ cm⁻³ concentration of phosphorus. Molybdenum(Mo), tungsten (W), MoSi₂ or WSi₂ film may be optionally formed on thissilicon film to form a multilayer film. The silicon film (or multilayerfilm) was patterned with a second photomask ○92 to obtain theconfiguration shown in FIG. 18(B). An NTFT gate electrode 909 and a PTFTgate electrode 921 were then formed. For example, as a gate electrode, aphosphorus-doped silicon 0.2 μm thick was formed and a molybdenum layer0.3 μm thick was formed thereon with a channel length of 10 μm.

As shown in FIG. 18(C), a photoresist 957 was formed using a photomask○93 , and boron was added using the ion implantation method at a dosageof 1 to 5×10¹⁵ cm⁻² for the PTFT source 918 and drain 920. Next, asillustrated in FIG. 18(D), a photoresist 961 was formed using aphotomask ○94 . Phosphorous was added using the ion implantation methodat a dosage of 1 to 5×10¹⁵ cm⁻² for an NTFT source 910 and drain 912.

Also, in the case where aluminum is used as the gate electrode material,after patterning with the second photomask ○92 , it is possible to formthe source and drain contact holes at positions closer to the gate byanodic oxidation of this surface of the patterned aluminum gateelectrode so that self-aligning construction can be applied. Therefore,the TFT characteristics can be further increased by improving themobility and decreasing the threshold voltage.

The doping above is made through a gate insulation film 954. However, inFIG. 18(B), using the gate electrodes 921, 909 as a mask the siliconeoxide on the silicone film may be removed, followed by the addition ofthe boron and phosphorous directly into the silicon film using the ionimplantation method.

Next, annealing was again conducted for 10 to 50 hours at 600° C. Theimpurities of the NTFT source 910 and drain 912, and the PTFT source918, and drain 920, were activated to form P⁺ and N⁺ regions. Thechannels 919 and 911 below the gate electrodes 921 and 909 were made ofa semi-amorphous semiconductor.

The entire manufacturing process for the C/TFT can thus be done withouthaving to apply a temperature above 700° C. in the self-aligning system.This makes it possible to use other materials other than expensivequartz as the substrate material. Accordingly, this embodiment of theinvention is very suitable for a liquid crystal display having a largepicture element.

The heat anneal process, shown in FIG. 18(A) and FIG. 18(D), wasperformed twice. However, the anneal process of FIG. 18(A) can beomitted, depending on the desired characteristics, and followed up withthe heat anneal process of FIG. 18(D), shortening the manufacturingtime. Also, in FIG. 18(E), the interlayer insulation layer 965 was madeof silicon oxide film using the sputtering method mentioned above.

This silicone oxide film can, however, also be formed using the LPCVDmethod or photo CVD method or normal pressure CVD method.

The thickness of the insulation layer was e.g. 0.2 to 0.6 μm.

Next, using photo mask ○45 , a window 966 for the electrodes was formed.Then, a layer of aluminum was formed over the entire structure using thesputtering method, and leads 971 and 972 and contacts 967, 968 were madeusing photo mask ○96 .

An organic resin film 969 for surface-flattening, e.g. a transparentpolyimide resin film was formed, and electrode openings were providedusing photomask ○97 .

Two TFTS were formed in a complementary structure in a picture elementof the liquid crystal display device as shown in FIG. 18(F), in whichthe output terminal of the TFTS was each connected to the (transparent)electrode of the picture element of the liquid crystal display device,forming the ITO (Indium Tin Oxide) by sputtering.

The electrode 917 was completed by etching through the photomask ○98 .

This ITO film was formed in the range from room temperature to 150° C.and finished by annealing at 200° C.to 400° C. in oxygen or atmosphere.The NTFT 913, the PTFT 922 and the transparent electrode 917 were thusprepared on a single glass substrate 950.

The electrical characteristics of the TFT obtained thus are as follows:

mobility in the PTFT: 20 cm² /Vs

Vth in the PTFT: -5.9 V

mobility in NTFT: 40 cm² /Vs

Vth in NTFT: 5.0 V

Another glass substrate provided with a transparent electrode over theentire surface thereof and the substrate fabricated according to theabove-described method were combined to form a liquid crystal cell. A TNliquid crystal material was injected into the liquid crystal cell. FIG.38 illustrates the positioning of the electrodes and the like for theliquid crystal display device according to this embodiment.

An NTFT 913 is provided at the intersection of a first signal line 905and a third signal line 903, and, in the same manner, an NTFT foranother picture element is provided at the intersection of the firstsignal line 905 and a third signal line 904. A PTFT is provided at theintersection of a second signal line 908 and the third signal line 903.Also, an NTFT for another picture element is provided at theintersection of another, adjacent first signal line 906 and the thirdsignal line 903, while in the same manner an NTFT is provided at theintersection of the first signal line 906 and the third signal line 904.

The NTFT 913 is connected to the first signal line 905 through a contacton the input terminal on the drain 910, and the gate 909 is connected toa signal line 903 which is formed of multilayer wiring. The outputterminal of the source 912 is connected to a picture element electrode917 through a contact.

The PTFT 922 is connected to the second signal line 908 through acontact on the input terminal on the drain 920, wherein the gate 921 isconnected to the signal line 903, and the output terminal of the source918 is connected to the picture element electrode 917 through a contactin the same way as in the NTFT.

Adjacently, another C/TFT which is connected to the same signal line 903is provided and the PTFT 922 of said another C/TFT is connected to asecond signal line 907, and the NTFT 913 of said another C/TFT isconnected to the first signal line 906.

One pixel comprising a picture element 923 formed from a transparentconducting film and a C/TFT is interposed between this pair of signallines 905 and 908. By repeating this type of configuration laterally andvertically, the 2×2 matrix can be expanded to form a large pictureelement liquid crystal display device of 640×480 or 1280×960 matrixes.

A special feature of this device is that the picture element electrode917 is set at three values of the liquid crystal potential V_(LC) byproviding a complementary configuration of two TFTs for one pictureelement.

Next, in order to form a second substrate, an ITO film (Indium TinOxide) was formed by sputtering on a substrate which was formed bylaminating a silicon oxide film to a thickness of 2000 Å on glass plateby the sputtering process. This ITO film was formed in the range fromroom temperature to 150° C. and finished by annealing at 200° C. to 400°C. in oxygen or atmosphere.

A polyimide precursor member was printed on the above-mentionedsubstrate using the offset method and fired for one hour at 350° C. inan oxygen-free atmosphere (for example, in a nitrogen atmosphere). Thepolyimide surface was then reformed using a commonly known rubbingmethod, so that a means for orienting the liquid crystal molecules in auniform direction in at least the initial stage was provided, wherebythe second substrate was completed.

Then, the liquid crystal composition having ferroelectricity wasinterposed between the first and second substrates, and the assembly wassealed around the periphery using an epoxy-type adhesive. A drive IC ofa TAB form was connected to a lead on the substrate and a polarizingplate was affixed to the outside to obtain a light-transmission type ofliquid crystal display device.

FIG. 14 shows the display for the A, E, and C picture elements when thedrive waveform shown in FIG. 16 is applied. In FIG. 14, darkness isexpressed by a dot. A clear gradated display is obtained, as shown inFIG. 14.

Embodiment 2

In this embodiment, a first substrate and a second substrate wereobtained using the same process as for the Embodiment 1. However, nopolyimide film for alignment was formed on the second substrate. Sincethis device was made for use in a video camera viewfinder, the pitch ofthe picture element was 60 μm, and a matrix 200 high×300 wide wasformed.

In this embodiment, a nematic liquid crystal composition was dispersedthroughout an acrylic organic resin to form a dispersed-type liquidcrystal display device. 62 wt % of the nematic liquid crystals wasdispersed throughout an acrylic resin denatured with anultraviolet-curable epoxy. This material was interposed between thefirst and second substrates, then cured by the application of a lightbeam from a UV light source with a 1000 mW output for 20 sec.

This display device was time-shared into 16 to provide a gradateddisplay, and each color had 16 gradations, to give a liquid crystaldisplay device which can display a total of 4096 colors. The drive waveform at that time is shown in FIG. 19.

In summary, a plurality of write-in entries (display frames) is providedin conventional gradated display methods. For example, 16 frames areutilized to provide a method for presenting a gradated display by acombination of their ON/OFF states. If, in a total 16 frames, eightframes are ON and the remaining eight frames are OFF, a gradated displayresults at a 50% transmission, which is the average transmittance inthis case.

If, however, four frames are ON and the remaining 12 frames are OFF, theaverage transmittance becomes 25% and a gradated display occurs at thistransmission.

When this conventional method is used there is a strong possibility ofthe number of frames less than the lowest confirmed number of frames, 30frames, which are the minimum number of frames that can be discerned bythe human eye. This is the main cause of a drop in the quality of thedisplay.

In this embodiment, where the frequency of the driver is increased forproviding a gradated display of the present invention, a gradateddisplay becomes possible preventing the actual frame frequency fromdecreasing. Therefore, the frequency never becomes lower than a visuallyconfirmed frequency, so that a drop in display quality does not occur,and a high quality picture can be provided.

By using the same type of process and drive method it is possible toprovide a word processor screen, a computer screen, or a device forprojecting a visual image display.

Embodiment 3

The liquid crystal electro-optical device utilized in this embodimenthas the circuit configuration shown in FIG. 10, namely, the circuitconfiguration of inverter type. FIG. 12 shows the layout of the actualelectrodes and the like corresponding to the circuit configuration ofFIG. 10. In order to simplify the explanation, the parts correspondingto a 2×2 matrix only are described. Also, the actual driving signalwaveform is shown in FIG. 9.

The process for forming a substrate for the liquid crystalelectro-optical device utilized in this embodiment is shown in FIGS.13(A) to (F). In accordance with the process shown in FIGS. 13(A) to(F), the substrate shown in FIG. 13(F) was formed in the same manner asin Embodiment 1. The substrate had the same structure as that Embodiment1 except that the location of a PTFT and an NTFT thereof was opposite tothat of Embodiment 1, as shown in FIGS. 10 and 13. With thus obtainedsubstrate, a light-transmission type liquid crystal electro-opticaldevice was completed as in Embodiment 1.

Embodiment 4

In this embodiment, a first substrate and a second substrate wereobtained using the same process as for Embodiment 3. However, anorientation film made of polyimide was not formed on the secondsubstrate. With these first and second substrates, a liquid crystalelectro-optical device for use in a video camera viewfinder was formedat a pitch of a picture element of 60 μm, and a matrix 200 high × 300wide in the same way as in Embodiment 2.

In this embodiment, where the frequency of the driver is increased forproviding a gradated display of the present invention, a gradateddisplay becomes possible preventing the actual frame frequency fromdecreasing. Therefore, the frequency never becomes lower than a visuallyconfirmed frequency, so that a drop in display quality does not occur,and a high quality picture can be provided.

By using the same type of process and drive method it is possible toprovide a word processor screen, a computer screen, or a device forprojecting a visual image display.

In the drive method of the present invention shown in FIG. 15 where theunit time t 225 for writing-in a picture element is devided into 16minimum units (a period of each minimum unit 227 is t/16), each color isdisplayed with 16 gradations, so that a display with 4096 colors ispossible in total.

Embodiment 5

In this embodiment, a liquid crystal display device with the circuitconfiguration shown in FIG. 3 is used.

FIG. 5 shows the layout of the actual electrodes and the likecorresponding to the circuit configuration of FIG. 3.

In order to simplify the explanation, the parts corresponding to a 4×4matrix (2×2 matrix) only are described.

Also, the actual driving signal waveform is shown in FIG. 1. Forsimplicity, the explanation of the signal waveform is also given for thecase of 4×4 matrix configuration.

The manufacturing process for the liquid crystal display device used inthis embodiment is shown in FIG. 6.

In FIG. 6(A), a silicon oxide film for a blocking layer 51 having athickness of 1000 to 3000 angstroms was formed on a glass substrate 50,using a magnetron RF (high frequency) sputtering method. The glasssubstrate 50 utilized was the one which was not expensive unlike quartzglass and was resistant to heat treatment up to 700° C. e.g. of 600° C.The conditions for the process are as follows:

Atmosphere: 100% oxygen

Film Formation Temperature: 150° C.

Output Power: 400-800 W

Pressure: 0.5 Pa

The film formation, using either quartz or single-crystal silicon for atarget, had a speed of 30 to 100 Å/min.

On the top surface thereof, a silicon film in an amorphous state havinga thickness of 500 to 5000 Å, e.g. 1500 Å, was formed as inEmbodiment 1. In the case of using low pressure CVD method to form theamorphous silicon film as in Embodiment 1, boron may be added at aconcentration of 1×10¹⁵ to 1×10¹⁸ cm⁻³ by the use of diborane during thefilm formation, in order to control the threshold voltage (Vth) of theNTFT.

Then, in the same manner as in Embodiment 1, the silicon film in anamorphous state was heat-annealed at an intermediate temperature of 450°to 700° C. for 12 to 70 hours under a non-oxide atmosphere. Then, a NTFTregion 13 was obtained from the silicon film by the use of a firstphotomask 1.

A silicon oxide film was then formed as a gate insulating film 54 in thethickness range of 500 to 2000 Å, for example, 1000 Å. This is preparedunder the same conditions as the silicone oxide film formed as ablocking layer. A small amount of fluorine may be added to the film forfixation of the sodium ion during the film formation.

When this operation was completed, a silicon film containing a 1 to5×10²¹ cm⁻³ concentration of phosphorus, or a multilayered filmcomprising the silicon film laminated thereon with molybdenum (Mo),tungsten (W), MoSi₂ or WSi₂ film was formed, which was then patternedwith a second photomask 2 to obtain the configuration shown in FIG.6(B). Phosphorus was added by ion implantation method at a dosage of 1to 5×10¹⁵ cm⁻² for a NTFT source 20 and drain 18, using the gateelectrode 9 as a mask.

Also, in the case where aluminum is used as the gate electrode material,after patterning with the second photomask 2, it is possible to form thesource and drain contact holes at positions closer to the gate by anodicoxidation of this surface of the patterned aluminum gate electrode sothat self-aligning construction can be applied. Therefore, the TFTcharacteristics can be further increased by improving the mobility anddecreasing the threshold voltage.

The above process is carried out through the gate insulating film 54.However, in FIG. 6(B), it is possible to remove the silicon oxide formedon the silicon film using the gate electrode 9 as a mask and then addthe phosphorus directly into the silicon film using the ion implantationmethod.

Next, annealing was again conducted for 10 to 50 hours at 600° C. Theimpurities were activated whereby the source 20 and drain 18 of the NTFTwere made N⁺ regions. A channel forming region 21 of semi-amorphoussemiconductor was formed below the gate electrode 9.

The entire manufacturing process for the NTFT can thus be done withouthaving to apply a temperature above 700° C. in spite of theself-aligning system. This makes it possible to use materials other thanexpensive ones such as quartz for the substrate material. Accordingly,this embodiment of the invention is very suitable for a liquid crystaldisplay having a large number of pixels.

The heat anneal process was carried out twice as shown in FIGS. 6(A) and(C). However, the anneal process of FIG. 6(A) may be omitted, dependingon the desired characteristics, and followed up with the heat annealprocess of FIG. 6(C), shortening the manufacturing time. Also, in FIG.6(D), the interlayer insulating layer 65 was made of silicon oxide filmusing the sputtering method mentioned above.

This silicon oxide film can, however, also be formed using the LPCVDmethod or photo CVD method or normal pressure CVD method. The thicknessof the film was e.g. 0.2 to 0.6 μm.

Next, using photo mask 3, an opening 66 for the electrodes was formed.Then, a layer of aluminum was formed on the entire surface bysputtering, and leads 71 and 72 and contacts 67, 68 were formed by usingphoto mask 4.

An organic resin film 69 for surface-flattening, e.g. a transparentpolyimide resin film was formed, and openings for electrodes wereprovided using a photomask 5.

The TFT was thus formed as shown in FIG. 6(F), and further an ITO(Indium Tin Oxide) film was formed by sputtering in order that theoutput terminal of the TFT was connected to a transparent electrode ofthe picture element of the liquid crystal display device by the ITO.

The electrode was completed by etching the ITO film through a photomask6, whereby a pixel electrode 17 and a contact 73 of the pixel electrodewith the drain electrode were completed. This ITO film was formed in therange from room temperature to 150° C. and annealed at 200° to 400° C.in oxygen or atmosphere.

The NTFT 13 and the transparent pixel electrode 17 were thus prepared onan identical glass substrate 50.

The electrical characteristics of the TFT thus formed are as follows:

mobility: 40 cm² /Vs

Vth: 5.0 V.

The first substrate was thus completed.

Then, a second substrate was manufactured in the same manner as thesecond substrate of Embodiment 1.

Then, the liquid crystal composition exhibiting ferroelectricity wasinterposed between the first and second substrates, and the assembly wassealed around the periphery using an epoxy-type adhesive. A drive IC ofa TAB form was connected to a lead on the substrate and a polarizingplate was affixed to the outside to obtain a light-transmission type ofliquid crystal display device.

FIG. 7 shows the display for the A, F, and I picture elements when thedrive waveform shown in FIG. 1 is applied. This figure shows that aclear gradated display is obtained.

Embodiment 6

In this embodiment, a first substrate and a second substrate wereobtained using the same process as for Embodiment 5. However, anorientation film made of polyimide was not formed on the secondsubstrate. With these first and second substrates, a liquid crystalelectro-optical device for use in a video camera viewfinder was formedat a pitch of a picture element of 60 μm and a matrix 200 high×300 widein the same way as in Embodiment 2.

In the drive method of the present invention shown in FIG. 8 where theunit time t for writing-in a picture element is devided into 16 minimumunits (a period of each minimum unit is t/16), each color is displayedwith 16 gradations, so that a display with 4096 colors is possible intotal.

In this embodiment, where the frequency of the driver is increased forproviding a gradated display of the present invention, a gradateddisplay becomes possible preventing the actual frame frequency fromdecreasing. Therefore, the frequency never becomes lower than a visuallyconfirmed frequency, so that a drop in display quality does not occur,and a high quality picture can be provided.

It is effective to improve the ability of gradated display over theconventional one by conducting the above-mentioned conventional methodfor gradated display together with the method for gradated display inaccordance with the present invention. The method of the presentinvention is to control the average voltage applied to a liquid crystalpixel, where complete response of liquid crystal is not required.

Conventionally, it was difficult to directly apply the voltages V_(b)and V_(c) shown in FIG. 2 to pixels. However, by changing the average ofthe voltage applied to pixel electrodes, an effect can be obtained as ifthe voltages V_(b) and V_(c) were directly applied to pixels.

In other words, the present invention is to provide a method forcontrolling liquid crystal which responds incompletely.

Although only N-channel field effect transistors are utilized in thisembodiment, P-channel field effect transistors may be utilized instead.

Embodiment 7

In this embodiment, a liquid crystal electro-optical device (imagedisplay device) with the circuit configuration shown in FIG. 21 isutilized as a television to be hanged on the wall. The TFTs utilizedtherein are made of polycrystal silicon subjected to laser annealing andare of stagger type.

Referring to FIG. 21, a reference numeral 700 designates a gateelectrode, 701 a source, 702 a drain, 703 a NMOSTFT, and 704 a pixelelectrode.

The layout of the actual electrodes and the like corresponding to thecircuit configuration in FIG. 21 is shown in FIG. 37. For simplifyingthe explanation, the parts corresponding to a 2×2 (or less) matrix onlyare described. Also, the reference numerals are made so as to correspondto those in FIG. 21.

The reference numeral 705 designates a lead contact and 706 a pixelcontact.

Further, the actual driving signal waveform is shown in FIG. 1. Forsimplicity, the explanation of the signal waveform is also given for thecase of 4×4 matrix configuration.

The manufacturing process for the liquid crystal display device used inthis embodiment is shown in FIG. 22.

In FIG. 22(A), a silicon oxide film for a blocking layer 801 having athickness of 1000 to 3000 angstroms was formed on a glass substrate 800,using a magnetron RF (high frequency) sputtering method. The glasssubstrate 800 utilized was the one which was not expensive unlike quartzglass and was resistant to heat treatment up to 700° C. e.g. of 600° C.The conditions for the process are as follows:

Atmosphere: 100% oxygen

Film Formation Temperature: 150° C.

Output Power: 400-800 W

Pressure: 0.5 Pa

The film formation, using either quartz or single-crystal silicon for atarget, had a speed of 30 to 100 Å/min.

On this silicon oxide film, a silicon film in an amorphous state wasformed. In the case of using plasma CVD method to form this amorphoussilicon film, the film formation temperature was from 250° C., e.g. 320°C. in this embodiment, and monosilane (SiH₄) was utilized. However,disilane (Si₂ H₆) or trisilane (Si₃ H₈) may be utilized instead ofmonosilane. The gas was inputted to a PCVD apparatus maintained at apressure of 3 Pa and a high frequency electric power was applied theretoat a frequency of 13.56 MHz, whereby the silicon film was deposited. Ahigh frequency electric power of 0.02 to 0.10 W/cm² was appropriate inthis case, and in this embodiment a high frequency electric power of0.055 W/cm² was applied. The flux of the monosilane was at 20SCCM andthe film formation rate under this condition was about 120 Å/min.

Boron may be added at a concentration of 1×10¹⁵ to 1×10¹⁸ cm⁻³ by usingdiborane during the film formation, in order to control the thresholdvoltage (Vth) of the NTFT.

Not only this plasma CVD method but also sputtering method and lowpressure CVD method can be utilized for forming the silicon film to be achannel region in a TFT.

In the case of using a sputtering method, the conditions were asfollows:

back pressure before sputtering: up to 1×10⁻⁵ Pa

target: a single crystal silicon

atmosphere: argon with hydrogen 20 to 80% by volume e.g. 20 volume % Arand 80 volume % H₂

the film forming temperature: 150 C.

frequency: 13.56 MHz

sputter output: 400 to 800 W

pressure: 0.5 Pa

If using the LPCVD method to form the silicon film, disilane (Si₂ H₆) ortrisilane (Si₃ H₈) was supplied to the CVD apparatus at a temperature100° to 200° C. less than the crystallization temperature, that is, 450°to 550° C., for example at 530° C. The pressure inside the reactionfurnace was 30 to 300 Pa. The film forming speed was 50 to 250 Å/minute.

With respect to the film formed by these methods, it is preferably thatthe oxygen concentration is 5×10²¹ cm⁻³ or less. In order to promotecrystallization of the film, it is desirable that the oxygenconcentration is 7×10¹⁹ cm⁻³ or less, preferably 1×10¹⁹ cm⁻³ or less.However, if the concentration is too low, the current leak in the OFFstate increases because of the back light. If the oxygen concentrationis too high, crystallization becomes difficult and the laser annealingtemperature must be increased or the annealing time lengthened. Siliconconcentration was assumed to be 4×10²² cm⁻³ and hydrogen concentrationwas 4×10²⁰ cm⁻³ which is equal to one atomic % of the siliconconcentration.

Also, to promote further crystallization at the source and drain, theoxygen concentration is adjusted to 7×10¹⁹ cm⁻³ or less, or, preferably,1×10¹⁹ cm⁻³ or less, and oxygen may be added by ion implantation to aconcentration range of 5×10²⁰ to 5×10²¹ cm⁻³, only to the channelforming regions of the TFTs which form the pixels.

By the above method, the silicon film 802 in an amorphous state wasformed to be 500 to 5000 Å in thickness, e.g. 1000 Å in this embodiment.

Then, photoresist 803 was formed in a pattern having openings therein onsource and drain regions as shown in FIG. 22(B) by the use of a mask 1.On this structure, a silicon film 804 to be an n-type activation layerwas formed by plasma CVD. The film formation temperature was 250° to350° C., specifically 320° C. in this embodiment. Monosilane (SiH₄) andphosphine (PH₃) of monosilane base at a concentration of 3% wereutilized. They were introduced into a PCVD apparatus maintained at apressure of 5 Pa and an electric power at a high frequency of 13.56 MHzwas inputted thereto, whereby the silicon film 804 was deposited. Thehigh frequency electric power of 0.05 to 0.20 W/cm² was appropriate inthis case, and in this embodiment an electric power of 0.120 W/cm² wasinputted.

The n-type silicon film formed by the above method had a specificelectric conductivity of about 2×10⁻¹ [Ωcm⁻¹ ]. The thickness thereofwas 50 Å. Then, source and drain regions 805 and 806 were formed bylift-off method. After that, an island region 807 for an N-channel thinfilm transistor was formed using a mask 82.

Subsequently, laser annealing to the source, drain, channel regions andlaser doping to the activation layer were carried out simultaneously bythe use of XeCl excimer laser. The threshold energy of this laser atthis moment was 130 mJ/cm². On the other hand, in order to melt thewhole film thickness, the energy of 220 mJ/cm² was necessary. However,if a laser having an energy more than 220 mJ/cm² was irradiated from thebeginning, hydrogen contained in the film would be discharged rapidly,resulting in the destroy of the film. For this reason, it is necessaryto firstly discharge the hydrogen at a low energy and then melt thefilm. In this embodiment, a laser at 150 mJ/cm² was irradiated todischarge hydrogen and then a laser at 230 mJ/cm² was irradiated tocrystallize the film.

It was supposed that when annealing the silicon film, crystallizationwas inclined to take place in a highly ordered state from the amorphousstructure, so that a crystal state was partly produced. Particularly, inthe regions where a relatively highly ordered state was produced justafter film forming of silicon, the tendency of crystallization to acrystal state was strong. A junction, however, took place due to thesilicon between these regions, so that the silicon attracted each other.

According to laser Raman spectrometry measurement of the annealedsilicon film, it was observed that a peak thereof was shifted from 522cm⁻¹ of the single crystal silicon to a lower frequency side. Theapparent grain diameter, when calculated using the half-value width, was50 to 500 Å. Actually, there were many of these highly crystallizedregions that made up clusters. Each cluster was joined to the other by asilicon junction (anchoring) forming a film.

Consequently, it was believed that the film could be said to havesubstantially no grain boundary (GB). The carriers could easily travelbetween the clusters through the anchored areas, so the mobility of thecarriers was higher than that of the polycrystalline silicon havingclear grain boundaries (GB). The electron mobility (μe) obtained was 15to 300 cm² /Vsec.

A silicon oxide film 808 was then formed as a gate insulating film inthe thickness range of 500 to 2000 Å, for example, 1000 Å. This isprepared under the same conditions as the silicon oxide film formed as ablocking layer. A small amount of fluorine may be added to this film forfixation of the sodium ion during film formation.

When this operation was completed, a silicon film containing phosphorusat 1 to 5×10²¹ cm⁻³ concentration, or a multilayered film comprising thesilicon film laminated thereon with molybdenum (Mo), tungsten (W), MoSi₂or WSi₂ film was formed. This film was patterned with a third photomask○83 to obtain the configuration shown in FIG. 22(E). A gate electrode809 was then formed. For example, as a gate electrode, aphosphorus-doped silicon 0.2 μm thick was formed and a molybdenum layer0.3 μm thick was formed thereon with a channel length of 7 μm.

Also, in the case where aluminum is used as the gate electrode material,after patterning with the third photomask ○83 , it is possible to formthe source and drain contact holes at positions closer to the gate byanodic oxidation of this surface of the patterned aluminum gateelectrode so that self-aligning construction can be applied. Therefore,the TFT characteristics can be further increased by improving themobility and decreasing the threshold voltage.

The entire manufacturing process for the TFT can thus be done withouthaving to apply a temperature above 400° C. This makes it possible touse, as the substrate, materials other than expensive materials such asquartz. Accordingly, this embodiment of the invention is very suitablefor a liquid crystal display having a large number of picture elements.

Then, a silicon oxide film was formed as an interlayer insulator 810 bysputtering method. In place of the sputtering method, LPCVD method,photo CVD method, and normal pressure CVD may be utilized for theformation of the silicon oxide film. The thickness of the layer was 0.2to 0.6 μm, for example. After that, an opening 811 for electrode wasformed using a forth photomask ○84 . On the entire surface of thisstructure, an aluminum film having a thickness of 0.3 μm was formed bysputtering method, and then a lead 812 and a contact 813 were formedusing a photomask ○83 . An organic resin for surface-flattering 814,e.g. a transparent polyimide resin was then applied on the top surface,and further an opening for an electrode was again formed using a sixthphotomask ○86 .

An ITO (Indium Tin Oxide) film of 0.1 μm thickness was formed on theentire surface of this structure by sputtering and was subsequentlypatterned into a pixel electrode 815 by using a seventh photomask ○87 .This ITO film was formed at room temperature to 150° C. and annealed at200° C. to 400° C. in an oxygen or an atmosphere.

The electrical characteristics of the thus formed TFT were as follows:

Mobility: 80 cm² /Vs

Vth: 5.0 V

In accordance with the foregoing method, the first substrate for aliquid crystal electro-optical device was completed.

The method for forming the second substrate is shown in FIG. 23.

A polyimide resin film, made of polyimide mixed with black pigment,having a thickness of 1 μm was formed on a glass substrate 500 by spincoat method and was then patterned into black stripes 501 by the use ofa first photomask 411.

Then, a film of polyimide resin mixed with red pigment having athickness of 1 μm was formed by spin coat method and was subsequentlypatterned into red color filters 502 by the use of a second photomask412.

In the same manner as the above, green color filters 503 were formed bythe use of a third photomask 413, and blue color filters 504 by the useof a fourth photomask 414.

During the formation of the filters, the filters were baked at 350° C.for 60 min. in an nitrogen atmosphere.

Subsequently, a transparent polyimide layer was formed as a levelinglayer 505 by spin coat method.

On the entire surface of the structure, an ITO film of 0.1 μm thicknesswas formed by sputtering and was patterned into a common electrode 506by the use of a fifth photomask 415. This ITO film was formed at roomtemperature to 150° C. and annealed at 200° to 300° C. in an oxygen oran atmosphere. Thus, the second substrate was completed.

A polyimide precursor material was printed on the above substrates byoff-set method and baked at 350° C. for 1 hour in an non-oxideatmosphere, e.g. in a nitrogen. Then the surface of the polyimide wassubjected to a known rubbing method, so as to provide a means fororienting liquid crystal molecules in a fixed direction in at least aninitial stage.

A nematic liquid crystal composition was interposed between the firstand second substrates and the periphery of the substrates was sealedwith an epoxy adhesive. An drive IC in TAB form and a PCB comprising acommon signal wiring and an electric potential wiring were connected tothe lead on the substrate, and a polarizing plate was affixed to theoutside whereby a light-transmission type liquid crystal electro-opticaldevice was obtained.

FIG. 24 shows the schematic configuration of the electro-optical deviceobtained in accordance with this embodiment.

The liquid crystal panel 1000 obtained by the above steps was combinedwith a back light device 1001 comprising three cool cathode tubes. Thena tuner 1002 for receiving TV electric waves was connected thereto, tothereby complete an electro-optical device. Since the electro-opticaldevice thus obtained had a flat form compared with the conventional CRTtype electro-optical device, it was possible to hang it on the wall andthe like.

Next, the configuration of a peripheral circuitry of the liquid crystalelectro-optical device is described with reference to FIG. 25.

The peripheral circuitry comprises a driver circuit 1103 connected toinformation signal side wires 1101 and 1102 which are connected to thematrix circuit of the liquid crystal electro-optical device. The drivercircuit 1103 is divided into two drive frequency systems. One of them isa data latch circuit system 1104 having a drive method same as theconventional method, where the main composition is a basic clock CLK1,1106 for transferring data 1105 by turns and 1-12 bits parallelprocessing is conducted. The other is the system composed in accordancewith the present invention, that is, it is composed of a flip flopcircuit 1108, a counter 1109 and a clock CLK2, 1107 for the independentfrequency from the data transfer frequency. Pulses are formed by thecounter 1109 so as to correspond to the gradated display datatransmitted from the data latch system 1104.

It is exactly this system which the present invention is characterizedby. That is, by utilizing two kinds of drive frequency, a clear digitalgradated display can be obtained without reducing frame numbers forrewriting a picture. Accordingly, occurrence of flicker and the like dueto the reduction of the frame number can be avoided.

On the other hand, in a driver circuit 1112 connected to scanning signallines 1110 and 1111, the electric potential transmitted from a voltagelevel 1113 is controlled by a flip flop circuit 1115 of a clock CLK 1114to supply address signals.

In the TFT obtained in accordance with this embodiment the mobility was80 cm² /Vs, so that drive frequency could be increased up to about 1MHz. Therefore, a gradated display of up to 42 gradations was possible,the gradation number being calculated by the following formula:

    1 MHz/(400*60)=42

where 1 MHz represents the drive frequency, 400 the duty number, and 60the frame number.

In the case of an analogue gradated display method, a gradated displayof 16 gradations was its limit due to the variation of the TFTcharacteristics. In the case of the digital gradated display method ofthe present invention, however, since the influence from the variationof the TFT characteristics is very little, a gradated display of 42gradations is possible. In the case of a color display, a colorful, finedisplay of 74,088 colors is possible.

Embodiment 8

This embodiment describes the manufacture of a video camera viewfinderutilizing a liquid crystal electro-optical device of 1 inch diagonal.

In this embodiment, a device utilizing amorphous TFTs in a 387×128matrix by low temperature processing was formed for a viewfinder. Themanufacturing method of the liquid crystal display device utilized inthis embodiment is explained below with reference to FIG. 26.

A silicon oxide film of 1000 to 3000 Å thickness was formed as ablocking layer 1201 on inexpensive glass 1200 such as sodalime glass bymagnetron RF (high frequency) sputtering method. The process conditionswere as follows:

Atmosphere: 100% oxygen

Film Formation Temperature: 15° C.

Output Power: 400-800 W

Pressure: 0.5 Pa.

Quartz or single-crystal silicon was utilized as a target, and the filmformation speed was 30 to 100 Å/min.

Then, a silicon film containing a 1 to 5×10²¹ cm⁻³ concentration ofphosphorus, or a multilayered film comprising the silicon film laminatedthereon with molybdenum (Mo), tungsten (W), MoSi₂ or WSi₂ film wasformed, which was then patterned with a first photomask ○21 to form agate electrode 1202 as in FIG. 26(A). In this embodiment, the channellength was 10 μm and as a gate electrode a phosphorus doped silicon filmof 0.2 μm thickness was formed and a molybdenum film of 0.3 μm thicknesswas formed further thereon.

In the case of utilizing aluminum (Al) for the gate electrode material,after patterning with the first photomask ○21 , it is possible toprevent the occurrence of cracks, voids in a channel region or aninsulating layer on the gate electrode by anodic oxidation of thissurface of the patterned aluminum gate electrode. Therefore, the TFTcharacteristics can be further increased by improving mobility anddecreasing the threshold voltage.

A silicon oxide film was then formed as a gate insulating film 1203 inthe thickness range of 500 to 2000 Å, e.g. 1000 Å. This was preparedunder the same conditions as the silicon oxide film formed as a blockinglayer. A small amount of fluorine may be added to this film for fixationof the sodium ions during the film formation.

An amorphous silicon film was then formed on this structure by plasmaCVD method. When forming a silicon film by plasma CVD, the temperaturewas maintained at e.g. 300° C. and monosilane (SiH₄) or disilane (Si₂H₆) was utilized. The gas was introduced into a PCVD apparatus and ahigh frequency electric power of 13.56 MHz was inputted thereto, wherebythe film was formed.

With respect to the film formed by the above method, it is preferablethat the oxygen concentration is 5×10²¹ cm⁻³ or less. If the oxygenconcentration is higher than this range, the mobility is decreased. Ifthe concentration is too low, the current leak in the OFF stateincreases because of the back light. For this reason, the concentrationis held in the range from 4×10¹⁹ to 4×10²¹ cm⁻³. Silicon concentrationwas assumed to be 4×10²² cm⁻³. Hydrogen concentration was 4×10²⁰ cm⁻³which is equal to one atomic % of the silicon concentration. Inaccordance with the above method, a silicon film in an amorphous statewas formed to be 500 to 5000 Å, e.g. 1500 Å thick.

Then, a resist film 1204 for forming a contact region by lift-off methodwas formed utilizing a second photomask ○22 , and on the top surface asilicon film 1205 to be an n-type activation layer was formed by plasmaCVD. The film formation temperature was in the range from 250° to 350°C.; at 320° C. in this embodiment. Monosilane and monosilane-basedphosphine (PH₃) at a concentration of 1% and hydrogen (H₂) wereintroduced at a ratio of 5:3:20 into a PCVD apparatus at a pressure of 5Pa and a high frequency electric field at 13.56 MHz was applied to formthe silicon film. At this moment, a high frequency electric power of0.05 to 0.20 W/cm² was appropriate, and an electric power of 0.120 W/cm²was utilized in this embodiment.

The thus formed silicon film 1205 to be an n-type activation layer had aspecific electric conductivity of about 2×10⁻¹ Ωcm⁻¹. The film thicknessthereof was 50 Å. Then, aluminum film 1206 of 3000 Å thickness wasformed for a lead and a contact electrode by sputtering. Unnecessaryportions of the aluminum film were removed by lift-off method to form asource region 1207 and a drain region 1208.

After forming each TFT 1209 in the form of island by the use of a thirdphotomask ○23 , an organic resin 1210 for surface flattering, e.g. atransparent polyimide resin, was applied as shown in FIG. 12(D), and anopening for electrode was again formed by the use of a photomask ○24 .

In order to connect the output end of the NTFT to one of transparentpixel electrodes of the liquid crystal device, an ITO (Indium Tin Oxide)film was formed by sputtering method. The ITO film was subjected toetching by using a photomask ○25 to form an electrode 1211. The ITO filmwas formed at room temperature to 150° C. and annealed at 200° C. to400° C. in an oxygen or an atmosphere. Thus, an NTFT 1209 and anelectrode 1211 made of a transparent conductive film were formed on anidentical glass substrate 1200. The electrical characteristics of thethus obtained TFT were as follows:

Mobility: 0.2 cm² /Vs

Vth: 5.3 V.

Next, in the same manner as in Embodiment 7, color filters and atransparent conductive film of ITO were formed to a thickness of 1000 Åon an insulating substrate to obtain a second substrate.

On the substrates, a polyimide precursor material was printed by off-setprinting and baked at 350° C. for 1 hour in an non-oxidation atmosphere,e.g. in nitrogen. The surfaces of the polyimide were then subjected to aknown rubbing treatment so as to provide a means for orienting liquidcrystal molecules in a fixed direction in at least an initial stage.Thus, the first and second substrates were completed.

Then a nematic liquid crystal composition was interposed between thefirst and the second substrates, and the periphery thereof was sealedwith an epoxy adhesive. Since the pitch of the leads on the substratewas so fine as 46 μm, the connection was carried out by the use of COGmethod. In this embodiment, leads were connected to gold bumps providedon an IC chip by means of a silver paradium resin of epoxy system, andthen an epoxy transformed acrylic resin was filled in the space betweenthe substrate and the IC chip for the purpose of fixing and enclosingthe IC chip and the substrates. Then a polarizing plate was affixed tothe outside thereof, whereby a light-transmission type liquid crystaldisplay device was obtained.

With the TFT in accordance with this embodiment, the mobility of 0.2 cm²/Vs could be obtained in spite of the amorphous state, and accordinglythe drive frequency could be increased to about 100 KHz. Therefore, agradated display having 13 gradations was possible, the gradation numberbeing calculated by the following formula:

    100 KHz/(128*60)=13

where 100 KHz represents a drive frequency, 128 duty number, and 60 aframe number. When carrying out the usual analogue gradated display witha liquid crystal electro-optical device of 50 mm square size (thesubstrate of which size is obtained by dividing 300 mm square substrateinto 36 plates) on which TFTs of 384×128=49,152 were formed, thevariation of the amorphous TFT characteristic was about ±10%, so that agraduated display of 8gradations was its limit. In the case of carryingout the digital graduated display method of the present invention, themethod was not affected by the variation of TFT characteristic so much,so that a gradated display of 13 gradations or more was possible. In thecase of a color display, a colorful, fine display of 2027 color waspossible.

Embodiment 9

This embodiment describes the manufacture of a projection type imagedisplay device as shown in FIG. 27(A-B)

In this embodiment, an image projecting part for a projection type imagedisplay device was assembled using three liquid crystal electro-opticaldevices 1300. Each of them had a 640×480 dot matrix, and 307,200 pixelswere formed within the size of 4 inch diagonal. The size of one pixelwas 127 μm square.

The projection type image display device is composed of three liquidcrystal electro-optical devices 1300 for three primary colors of light,i.e. red, green, and blue respectively, a red color filter 1301, a greencolor filter 1302, a blue color filter 1303, reflection boards 1304, ametal halide light source 1307 of 150 W, and an optical system for focus1308.

The substrate of the liquid crystal electro-optical device utilized foran electro-optical device of this embodiment was the one having the NMOSconfiguration and a matrix circuitry. A device comprising high mobilityTFTs formed by low temperature process was utilized to compose theprojection type liquid crystal electro-optical device.

The manufacturing method for the liquid crystal display device utilizedin this embodiment is explained hereinafter with reference to FIG. 28.

A silicon oxide film of 1000 to 3000 Å thickness was formed as ablocking layer 1401 on glass 1400 by magnetron RF (high frequency)sputtering as shown in FIG. 28(A). The glass was the one which was notexpensive unlike quartz glass or so and was resistant to thermaltreatment at not higher than 700° C., e.g. about 600° C. The processconditions of the film formation were the same as those for the siliconoxide film as a blocking layer in Embodiment 1.

On the silicon oxide film, a silicon film in an amorphous state wasformed to be 500 to 5000 Å thick, e.g. 1500 Å thick, in the same manneras the case of the silicon film in an amorphous state in Embodiment 1.

As in Embodiment 1, the silicon film in an amorphous state was thenannealed at an intermediate temperature of 450° C. to 700° C. for 12 to70 hours in a non-oxide atmosphere.

Then the silicon film was subjected to photo etching by the use of afirst photomask ○31 to form a region 1402 for TFT (having a channelwidth of 20 μm), as shown in FIG. 28(A).

A silicon oxide film of 500 to 2000 Å thickness, e.g. 1000 Å thickness,was then formed as a gate insulating film 1403. The formation conditionsthereof were the same as those for the silicon oxide film as a blockinglayer. A small amount of fluorine may be added during the film formationfor fixation of sodium ions.

Then, a silicon film containing a 1 to 5×10²¹ cm⁻³ concentration ofphosphorus, or a multilayered film comprising the silicon film laminatedthereon with molybdenum (Mo), tungsten (W), MoSi₂ or WSi₂ film wasformed, which was subsequently patterned with a second photomask ○32 toform a gate electrode 1404 as in FIG. 28(B). In this embodiment, achannel length was made 10 μm, and as a gate electrode the phosphorusdoped silicon of 0.2 μm thickness was formed and further a molybdenumfilm of 0.3 μm thickness was laminated thereon. In FIG. 28(C),phosphorus was added by ion implantation method at a dosage of 1 to5×10¹⁵ cm⁻² to form a source 1405 and a drain 1406.

In the case of utilizing aluminum (A1) as a gate electrode material,after patterning with the second photomask ○32 , it is possible to formthe source and drain contact holes at positions closer to the gate byanodic oxidation of this surface of the patterned aluminum gateelectrode so that self-aligning construction can be applied. Therefore,the TFT characteristics can be further increased by improving themobility and decreasing the threshold voltage.

Next, heat annealing was again carried out at 600° C. for 10 to 50hours. Impurities in the source 1405 and the drain 1406 were activatedto make the source and drain N⁺ regions. A channel forming region 1407of semi-amorphous semiconductor was formed below the gate electrode1404.

The entire manufacturing process for the NTFT can thus be done withouthaving to apply a temperature above 700° C. in spite of theself-aligning system. This makes it possible to use materials other thanexpensive ones such as quartz for substrates. Accordingly, thisembodiment of the invention is very suitable for a liquid crystaldisplay having a large number of pixels.

In this embodiment, the heat annealing process was carried out twice asshown in FIGS. 28(A) and (C). However, the anneal process of FIG. 28(A)may be omitted, depending on the desired characteristics, and followedup with the heat anneal process of FIG. 28(C), shortening themanufurturing time. In FIG. 28(D), an interlayer insulating layer 1408was made of silicon oxide film using the sputtering method mentionedabove.

This silicon oxide film can, however, be formed using the LPCVD method,photo CVD method or normal pressure CVD method.

The thickness of the insulating layer was e.g. 0.2 to 0.6 μm.

Next, using photomask ○33 , an opening 1409 for the electrodes wasformed. Then, a layer of aluminum was formed over the entire structureusing the sputtering method, and a lead 1410 and a contact 1411 wereformed using photomask ○34 .

An organic resin film 1412 for surface-flattering, e.g. a transparentpolyimide resin film was formed, and an opening for an electrode wasformed using photomask ○35 .

In order to connect the output terminal of the NTFT to the transparentelectrode of the pixel of the liquid crystal device, an ITO (Indium TinOxide) film was formed by sputtering. The ITO film was etched by the useof a photomask ○36 to form an electrode 1413. The ITO film was formed atroom temperature to 150° C. and annealed at 200° to 400° C. in an oxygenor an atmosphere.

Thus, the NTFT 1402 and the electrode 1413 of a transparent conductivefilm were formed on an identical glass substrate 1400. The electricalcharacteristics of the obtained TFT were as follows:

Mobility: 120 cm² /Vs

Vth: 5.0 V.

A schematic view of the structure is shown in FIG. 29. On the substrate(1500 in FIG. 29) mentioned above, a mixture of 10 μm thickness wasformed by die-cast method, the mixture comprising fumaric acid polymericresin and nematic liquid crystal both dissolved at a ratio of 65:35 in acommon solvent, xylene. Then the above structure was heated at 120° C.for 180 min. in a nitrogen atmosphere to remove the solvent, whereby aliquid crystal dispersion layer 1501 was formed. It was found thattact-time could be shortened by making the pressure a bit lower than theatmospheric pressure at the moment.

Then, an ITO film was formed by sputtering to obtain an opposedelectrode 1502. This ITO film was formed at room temperature to 150° C.A transparent silicon resin was then applied to be 30 μm thick byprinting method and was baked at 100° C. for 30 min. to thereby obtain aliquid crystal electro-optical device.

The configuration and the function of the driver IC utilized in thisembodiment is the same as that in Embodiment 7.

When a usual analogue gradated display was carried out with a liquidcrystal electro-optical device where 307,200 TFTs in 640×480 dot matrixwere formed within 300 mm square, the variation in TFT characteristicswas as large as about ±10%, so that a gradated display of up to 16gradations was its limit. In the case of the TFTs formed in thisembodiment, however, since the drive frequency can be increased up to2.5 MHz, a gradated display of up to 86 gradations is possible, thegradation number being calculated by the following formula:

    2.5 MHz/(480×60)=86

where 2.5 MHz represents the drive frequency, 480 the number of scanninglines, and 60 the number of frames.

In the case of the digital gradated display method in accordance withthis embodiment, the method is not affected by the variation in TFTcharacteristics so much, so that a gradated display of 86 gradations ispossible. With regard to a color display, a colorful, fine displayhaving 262,144 colors can be obtained.

A conventional television set utilizing a liquid crystal display having16 gradations is not suitable for displaying a natural landscape, forexample a `lock` of one color, since a hollow (uneven) surface of the`lock` of one color should be displayed by subtly different colors inorder to express a variety of shades of the hollow (uneven) surface insunshine. However, the gradated display in accordance with the presentinvention makes it possible to project a picture, e.g. a `lock` of onecolor, with variations of fine tone.

This liquid crystal electro-optical device is applicable not only to afront type projection TV as shown in FIG. 27 but also a rear typeprojection TV.

Embodiment 10

This embodiment shows the manufacture of an electro-optical device for aportable computer utilizing a liquid crystal dispersion type displaydevice of reflection type as shown in FIG. 30.

The first substrate utilized in this embodiment was formed by the samesteps as in Embodiment 7.

This embodiment will be explained below utilizing the liquid crystalelectro-optical device shown in FIG. 29.

Fumaric acid polymeric resin and a nematic liquid crystal mixed with ablack pigment at 15% were dissolved in a common solvent, xylene at aratio of 65:35. This solution was formed to a thickness of 10 μm on thesubstrate 1500 by die-cast method and was then heated at 120° C. for 180min. in a nitrogen atmosphere to remove the solvent, whereby a liquidcrystal dispersion layer 1501 was obtained.

Then, an ITO film was formed by sputtering to obtain an opposedelectrode 1502. This ITO film was formed at room temperature to 150° C.Then, a silicon resin of white color of 55 μm thickness was applied onthe rear surface by printing method and baked at 100° C. for 90 min toobtain a liquid crystal electro-optical device.

By utilizing the black pigment as the above, it became possible todisplay black color displayed when the light was dispersed (i.e. when noelectric fields were applied) and also white color when the light wastransmitted (i.e. when electric fields were applied), whereby a displayas if characters were written on a paper could be obtained.

Alternatively, it was possible to display white color when the light wasdispersed and black color when the light was transmitted, without mixingthe black pigment. In this case, however, it was necessary to make therear surface black. A display as if characters were written on a papercould be also obtained.

Embodiment 11

This embodiment shows the manufacture of a television set to be hangedon the wall utilizing a liquid crystal display device having the circuitconfiguration shown in FIG. 31. TFTs utilized therein are made ofpolycrystalline silicon subjected to laser annealing and in staggertype.

FIG. 32 shows the layout of the actual electrodes and the likecorresponding to the circuit configuration in FIG. 31.

In order to simplify the explanation, the parts corresponding to a 2×2(or less) matrix only are shown therein.

Also, the actual driving signal waveform is shown in FIG. 16. Forsimplicity, the explanation of the signal waveform is also given for thecase of 2×2 matrix configuration.

The manufacturing process for a liquid crystal panel utilized in thisembodiment is explained with reference to FIG. 33.

In FIG. 33(A), a silicon oxide film for a blocking layer 651 having athickness of 1000 to 3000 Å was formed on a glass substrate 650, using amagnetron RF (high frequency) sputtering method. The glass substrate 650was made of inexpensive materials glass capable of withstanding heattreatment up to 700° C., e.g. about 600° C.

The process conditions were the same as those for the silicon oxide filmas a blocking layer in Embodiment 7.

On the blocking layer 651, a silicon film 652 in an amorphous state wasformed to be 500 to 5000 Å thick, e.g. 1000 Å thick, in the same manneras that for the silicon film in an amorphous state in Embodiment 7.

As shown in FIG. 33(B), a photoresist pattern 653 with source and drainregions opened was formed using a mask ○P1 . Then, a silicon film to bean n-type activation layer was formed thereon by plasma CVD method. Thefilm formation temperature was maintained at 250° C. to 350° C.,specifically 320° C. in this embodiment. Monosilane (SiH₄) andmonosilane-based phosphine (PH₃) at a concentration of 3% wereintroduced into a PCVD apparatus at a pressure of 5 Pa and a highfrequency electric power at 13.56 MHz was inputted thereto, to therebyform the silicon film. A high frequency electric power of 0.05 to 0.20W/cm² was appropriate, and a high frequency electric power of 0.120W/cm² was inputted in this embodiment. The specific electricconductivity of the n-type silicon film thus obtained was about 2×10⁻¹Ωcm³¹ 1. The film was 50 Å.

On the other hand a photoresist pattern 654 with source and drainregions opened was formed using a mask ○P2 , as shown in FIG. 33(C).Then, a silicon film to be a p-type activation layer was formed thereonby plasma CVD method. The film formation temperature was maintained at250° C. to 350° C., specifically 320° C. in this embodiment. Monosilane(SiH₄) and monosilane-based diborane (B₂ H₆) at a concentration of 2%were introduced into a PCVD apparatus at a pressure of 4 Pa and a highfrequency electric power at 13.56 MHz was inputted to thereby form thesilicon film. A high frequency electric power of 0.05 to 0.20 W/cm² wasappropriate, and a high frequency electric power of 0.080 W/cm² wasinputted in this embodiment. The specific electric conductivity of thep-type silicon film thus obtained was about 1×10⁻¹ Ωcm⁻¹. The filmthickness was 50 Å.

Then, source and drain regions 655, 656 and 657, 658 were formed bylift-off method. After that, an island region 663 for an N-channel typethin film transistor and an island region 664 for a P-channel type thinfilm transistor were formed using a mask ○P3 662.

Subsequently, laser annealing to the source, drain, channel regions andlaser doping to the activation layers were carried out simultaneously bythe use of XeCl excimer laser in the same way as in Embodiment 7. Anelectron mobility (μe) of 15 to 300 cm² /Vsec and a hole mobility (μe)of 5 to 100 cm² /Vsec can be obtained.

A silicon oxide film of 500 to 2000 Å thickness, e.g. 1000 Å thickness,was then formed as a gate insulating film. This was prepared under thesame conditions as the silicon oxide film formed as a blocking layer. Asmall amount of fluorine may be added to this film for fixation of thesodium ions during film formation.

When this operation was completed, a silicon film containing phosphorusat 1 to 5×10²¹ cm⁻³ concentration, or a multilayered film comprising thesilicon film laminated thereon with molybdenum (Mo), tungsten (W), MoSi₂or WSi₂ film was formed, which was patterned with a fourth photomask 669to obtain the configuration shown in FIG. 33(E). Gate electrodes 666 and667 were then formed. For example, for gate electrodes aphosphorus-doped silicon layer 0.2 μm thick was formed and a molybdenumlayer 0.3 μm thick was formed thereon with a channel length of 7 μm.

Also, in the case where aluminum is used as the gate electrode material,after patterning with the fourth photomask 669, it is possible to formthe source and drain contact holes at positions closer to the gates byanodic oxidation of this surface of the patterned aluminum gateelectrode so that self-aligning construction can be applied. Therefore,the TFT characteristics can be further increased by improving themobility and decreasing the threshold voltage.

The entire manufacturing process for the C/TFT can thus be done withouthaving to apply a temperature above 400° C. This makes it possible touse, as the substrate, materials other than expensive materials such asquartz. Accordingly, this embodiment of the invention is very suitablefor a liquid crystal display having a large picture plane.

In FIG. 33(F), a silicon oxide film was formed as an interlayerinsulator 668 by sputtering method. In place of the sputtering method,LPCVD method, photo CVD method, and normal pressure CVD may be utilizedfor the formation of the silicon oxide film. The thickness of the layerwas 0.2 to 0.6 μm, for example. After that, an opening 679 for electrodewas formed using a fifth photomask 670. On the entire surface of thisstructure, an aluminum film having a thickness of 0.3 μm was formed bysputtering method, and then a lead 674 and a contact 673 were formedusing a sixth photomask 676.

A silicon oxide film was again formed as an interlayer insulating layer680 by the above mentioned sputtering method. Instead of the sputteringmethod, the LPCVD method, photo CVD method, and normal pressure CVDmethod may be utilized for forming the silicon oxide film. The siliconoxide film was then patterned using a seventh photomask 681. Then, onthe entire surface an aluminum film of 0.3 μm thickness was formed bysputtering. A lead 683 and a contact 684 were then formed using aneighth photomask 682.

An organic resin for surface-flattering 685, e.g. a transparentpolyimide resin was then applied on the top surface, and further anopening for electrode was again formed using a ninth photomask 686.

An ITO film of 0.1 μm thickness was formed on the entire surface of thisstructure by sputtering and was subsequently patterned into pixelelectrodes 688 by the use of a tenth photomask 687. This ITO film wasformed at room temperature to 150° C. and annealed at 200° C. to 400° C.in an oxygen or an atmosphere.

The electrical characteristics of the thus formed NTFT and PTFT were asfollows:

    ______________________________________                                                NTFT . . .                                                                            Mobility: 80 cm.sup.2 /Vs                                                     Vth: 5.0 V                                                            PTFT . . .                                                                            Mobility: 30 cm.sup.2 /Vs                                                     Vth: 5.5 V.                                                   ______________________________________                                    

In accordance with the foregoing method, the first substrate for aliquid crystal electro-optical device was completed.

A second substrate for the liquid crystal electro-optical device wasformed in the same manner as in Embodiment 7.

A nematic liquid crystal composition was interposed between the firstand second substrates and the periphery of the substrates was sealedwith an epoxy adhesive. An driver IC in TAB form and a PCB comprising acommon signal wiring and a potential wiring were connected to the leadon the substrate, and a polarizing plate was affixed to the outsidewhereby a light-transmission type liquid crystal electro-optical devicewas obtained.

The structure of the electro-optical device obtained in accordance withthis embodiment is the same as that in Embodiment 7 and schematicallyillustrated in FIG. 24.

Next, the configuration of a peripheral circuitry of the liquid crystalelectro-optical device is described with reference to FIG. 34.

A driver circuit 352 is connected to information signal side wires 350and 351 which are connected to the matrix circuit of the liquid crystalelectro-optical device. The driver circuit 352 is divided into two drivefrequency systems. One of them is a data latch circuit system 353 havinga drive method same as the conventional method, where the maincomposition is a basic clock φH 355 for transferring data 356 by turnsand 1-12 bits parallel processing is conducted. The other is the systemcomposed in accordance with the present invention, that is, it iscomposed of a magnitude comparator circuit 358, a buffer 360 for paneldrive and a clock CLK 357 for the independent frequency of the datatransfer frequency. Pulses are formed by the counter 358 so as tocorrespond to the gradated display data transmitted from the data latchsystem 353.

It is exactly this system which the present invention is characterizedby. That is, by utilizing two kinds of drive frequency, a clear digitalgradated display can be obtained without reducing frame numbers forrewriting a picture. Accordingly, occurrence of flicker and the like dueto the reduction of the frame number can be avoided.

FIG. 35 is a photograph of oscilloscope showing input signal waveformsinputted to and output signal waveforms outputted from the C/TFTobtained in this embodiment. In FIGS. 35(A) to (D), drive frequency ofinput signals is raised as 5 KHz, 50 KHz, 500 KHz, and 1 MHz. As isapparent from FIG. 35(D), even at 1 MHz, output signal waveforms do notbecome gentle so much and fully useful output signals can be obtained.

The number of gradations of a gradated display can be calculated bydividing the drive frequency by duty number and frame number. In thecase of the drive frequency of 1 MHz, a gradated display of 42gradations (calculated by dividing 1 MHz by 400 and 60) can be obtained.

In the case of an analogue gradated display method, a gradated displayof 16 gradations was its limit due to the variations in TFTcharacteristics. In the case of the digital gradated display method ofthe present invention, however, since the method is not affected by thevariations in TFT characteristics so much, a gradated display of up to42 gradations is possible. With regard to a color display, a colorful,fine display of 74,088 colors can be obtained.

Embodiment 12

This embodiment shows the manufacture of a video camera viewfinderutilizing a liquid crystal electro-optical device of 1 inch diagonal.

In this embodiment, a first substrate with 387×128 pixel configurationwas prepared by the same process as in Embodiment 11. Also, a secondsubstrate was prepared by providing color filters and a transparentconductive film ITO to a thickness of 1000 Å on a substrate made ofinsulator by the same process as in Embodiment 11.

On the above substrates, a polyimide precursor material was printed byoff-set method, and subsequently the substrates were baked at 350° C.for 1 hour in an non-oxidizing atmosphere, e.g. in a nitrogen. Then, thesurfaces of the polyimide films were subjected to a know rubbingtreatment, whereby the first and second substrate provided with meansfor orientating liquid crystal molecules in one fixed direction in atleast an initial stage were obtained.

A nematic liquid crystal composition was interposed between the abovefirst and second substrates and the periphery of the substrates wassealed with an epoxy adhesive. Since the pitch of the leads on thesubstrates was as fine as 46 μm, connection was conducted by COG method.In this embodiment, leads were connected to gold bumps provided on an ICchip by means of a silver paradium resin of epoxy system, and then anepoxy transformed acrylic resin was filled in the space between the ICchip and the substrates for the purpose of fixing and enclosing the ICchip. Then a polarizing plate was affixed to the outside thereof,whereby a light-transmission type liquid crystal display device wasobtained.

Since the channel length was 5 μm in the TFT of this embodiment, thedrive frequency could be raised up to about 2 MHz. Hence, in accordancewith the division of 2 MHz by 128 and 60, 260 gradations, approximately256 gradations were possible in a gradated display. When carrying outthe usual analogue gradated display with a liquid crystalelectro-optical device of 50 mm square size (the substrate of which sizeis obtained by dividing 300 mm square-sized substrate into 36 plates) onwhich TFTs of 384×128=49,152 were formed, the variation in the amorphousTFT characteristic was about ±10%, so that a gradated display of 16gradations was its limit. In the case of carrying out the digitalgradated display method of the present invention, the method was notaffected by the variation in TFT characteristic so much, so that agradated display of 256 gradations or more was possible. In the case ofa color display, a colorful, fine display of 16,777,216 colors waspossible.

Embodiment 13

This embodiment describes the manufacture of a projection type imagedisplay device as shown in FIG. 27.

In this embodiment, an image projecting part for a projection type imagedisplay device was assembled using three liquid crystal electro-opticaldevices 1300. Each of them had a 640×480 dot matrix, and 307,200 pixelswere formed within the size of 4 inch diagonal. The size of one pixelwas 127 μm square.

The projection type image display device is composed of three liquidcrystal electro-optical devices 1300 for three primary colors of light,i.e. red, green, and blue respectively, a red color filter 1301, a greencolor filter 1302, a blue color filter 1303, reflection boards 1304, ametal halide light source 1307 of 150 W, and an optical system for focus1308.

The substrate of the liquid crystal electro-optical device utilized foran electro-optical device of this embodiment was the one having C/TFTconfiguration and a matrix circuitry. A device comprising high mobilityTFTs formed by low temperature process was utilized to compose theprojection type liquid crystal electro-optical device.

The manufacturing method for the liquid crystal display device utilizedin this embodiment is explained hereinbelow with reference to FIG. 36.

In FIG. 36(A), a silicon oxide film of 1000 to 3000 Å thickness wasformed as a blocking layer 602 on glass 601 by a magnetron RF (highfrequency) sputtering. The glass was the one which was not expensiveunlike quartz glass or so and was resistant to thermal treatment at nothigher than 700° C., e.g. about 600° C. The process conditions of thefilm formation were the same as those for the silicon oxide film as ablocking layer in Embodiment 1.

On the silicon oxide film, a silicon film 603 in an amorphous state wasformed to be 500 to 5000 Å thick, e.g. 1500 Å thick, in the same manneras the case of the silicon film in an amorphous state in Embodiment 1.

As in Embodiment 1, the silicon film in an amorphous state was thenheat-annealed at an intermediate temperature of 450° to 700° C. for 12to 70 hours in an non-oxide atmosphere.

A silicon oxide film 604 of 500 to 2000 Å thickness, e.g. 1000 Åthickness, was then formed as a gate insulating film. The formationconditions thereof were the same as those for the silicon oxide film asa blocking layer. A small amount of fluorine may be added during thefilm formation for fixation of sodium ions.

Then, a silicon film containing a 1 to 5×10²¹ cm⁻³ concentration ofphosphorus, or a multilayered film comprising the silicon film laminatedthereon with molybdenum (Mo), tungsten (W), MoSi₂ or WSi₂ film wasformed, which was subsequently patterned with a first photomask ○41 asin FIG. 36(B). In this embodiment, a molybdenum film was formed to athickness of 0.3 μm as a gate electrode with a channel length of 10 μm.In the patterning, the gate electrodes were overetched 77 at about 3 μm.Then, a positive photoresist 607 was applied on the entire surface ofthe substrate.

After the application, exposure and development were carried out fromthe rear side of the substrate, using a photomask ○42 to thereby obtaina resist 608. Then, an n-type layer was deposited by sputtering. Bysubsequently removing the resist 608 by lift-off method, theconfiguration shown in FIG. 36(D) was obtained.

In the same manner, after a positive photoresist was applied on theentire surface of the substrate, exposure and development were carriedout from the rear side of the substrate using a photomask ○43 to therebyobtain a resist 610. Then a p-type layer was deposited by sputtering. Byremoving the resist 610 by lift-off method, the configuration shown inFIG. 36(E) was obtained.

The substrate was again heat-annealed at 600° C. for 10 to 50 hours,whereby impurities in sources 612, 614 and drains 613, 615 wereactivated to be N⁺ or P⁺ type. Channel formation regions 618 and 619 ofsemi-amorphous semiconductor was formed below gate electrodes 616 and617.

The entire manufacturing process for the C/TFT can thus be done withouthaving to apply a temperature above 700° C. in the self-aligning system.This makes it possible to use materials other than expensive ones suchas quartz as the substrate material. Accordingly, this embodiment of theinvention is very suitable for a liquid crystal display having a largepicture plane.

The heat anneal process, shown in FIG. 36(A) and FIG. 36(E), wasperformed twice. However, the anneal process in FIG. 36(A) can beomitted, depending on the desired characteristics, and followed up withthe heat anneal process of FIG. 36(E), shortening the manufacturingtime.

In FIG. 36(F), a silicon oxide film was formed as an interlayerinsulator 620 by the sputtering method mentioned above. This siliconoxide film may be formed by the LPCVD method, photo CVD method, ornormal pressure CVD method, instead. The thickness of the insulator wase.g. 0.2 to 0.6 μm. Next, using a photomask ○44 , openings 621 for theelectrodes were formed. Then, a layer of aluminum was formed on theentire structure using the sputtering method, and a lead 622 and acontact 623 were formed using a photomask ○45 , as shown in FIG. 36(F).

An organic resin film 624 for surface-flattering, e.g. a transparentpolyimide resin film was formed, and electrode an opening for anelectrode was provided using a photomask ○46 .

In order to connect the output terminal of the C/TFT to the(transparent) electrode of the pixel of the liquid crystal displaydevice, an ITO film was formed by sputtering.

The electrode 625 was completed by etching through a photomask ○47 .

This ITO film was formed in the range from room temperature to 150° C.and annealed at 200° C. to 400° C. in oxygen or atmosphere. The NTFT626, PTFT 627, and the transparent electrode 625 were thus prepared onan identical glass substrate 601.

The electrical characteristics of the TFTs thus obtained are as follows:

    ______________________________________                                               NTFT . . .                                                                            Mobility: 120 cm.sup.2 /Vs                                                    Vth: 5.0 V                                                            PTFT . . .                                                                            Mobility: 50 cm.sup.2 /Vs                                                     Vth: 5.5 V.                                                    ______________________________________                                    

The substrate 1500 shown in FIG. 29 was obtained by the foregoingprocess.

A liquid crystal dispersion layer 1501 was formed on the substrate 1500in the same manner as in Embodiment 9, as shown in FIG. 29. Further anopposed electrode 1502 was formed thereon as in Embodiment 9. Then, atransparent silicon resin of 30 μm thickness was applied on the topsurface of the structure by printing method and was baked at 100° C. for30 min. to thereby obtain a liquid crystal electro-optical device.

The configuration and the function of the driver IC utilized in thisembodiment are the same as in Embodiment 11.

When a usual analogue gradated display was carried out with a liquidcrystal electro-optical device where 307,200 TFTs in 640×480 dot matrixwere formed within 300 mm square, the variation in TFT characteristicswas as large as about ±10%, so that a gradated display of up to 16gradations was its limit. In the case of the TFTs formed in thisembodiment, however, since the drive frequency can be increased up to2.5 MHz, a gradated display of up to 86 gradations is possible, thegradation number being calculated by the following formula:

    2.5 MHz/(480×60)=86

where 2.5 MHz represents the drive frequency, 480 the number of scanninglines, and 60 the number of frames.

With regard to a color display, a colorful, fine display having 262,144colors can be obtained.

This liquid crystal electro-optical device is applicable not only to afront type projection TV shown in FIG. 27 but also to a rear typeprojection TV.

Embodiment 14

This embodiment shows the manufacture of an electro-optical device for aportable computer utilizing a liquid crystal dispersion type displaydevice of reflection type as in FIG. 30.

A first substrate 1500 utilized in this embodiment was formed in thesame manner as in Embodiment 11.

As shown in FIG. 29, a liquid crystal dispersion layer 1501, a counterelectrode 1502 on the layer 1501, and a transparent silicon resin on theelectrode 1502 were provided on the substrate 1500 as in Embodiment 10.The silicon resin was subsequently baked at 100° C. for 90 min. tothereby obtain a liquid crystal electro-optical device.

In the present invention, a gradated display is provided using a displaydrive system with the display timing related to the unit time t forwriting-in a picture element and to the time F for writing-in onepicture, wherein, by time-sharing the signal during a write-in of timet, without changing the time F, a clear gradated display controlled bydigital can be obtained. Compared with the gradated display method usinga plurality of frames, a display of high quality is possible without thedecrease of display frequency by the digital gradated display method ofthe present invention.

Instead of a conventional analogue gradated display, the presentinvention provides a digital gradated display with two kinds of drivefrequencies being independent of each other. In the case of utilizing aliquid crystal electro-optical device in 640×400 dot matrix,conventionally it was very difficult to form all the 256,000 TFTswithout variations in characteristic, and taking the actual productivityand yields into consideration a gradated display of 16 gradations wasits limit. On the other hand, in order to make clear the applied voltagelevel, a reference voltage value is inputted, instead of an analoguevalue, as a signal from the controller side in the present invention. Bycontrolling by a digital value the timing to connect the referencesignal to TFT, the voltage applied to the TFT is controlled, whereby thevariation in TFT characteristics is covered. Hence, a clear digitalgradated display is possible in accordance with the present invention.

The use of two kinds of drive frequencies makes it possible to obtain aclear digital gradated display without changing the number of frames forrewriting a picture, whereby the occurrence of flicker and the like dueto the decrease of the frame number can be avoided.

What is claimed is:
 1. An electro-optical device comprising:a pair ofsubstrates; an electro-optical modulating layer provided between saidsubstrates; matrix lines comprising column control lines and row controllines provided on one of said substrates; a plurality of pixelelectrodes provided on said one of said substrates; a plurality oftransistors provided on said one of said substrates, each of thetransistors being connected to corresponding one of said pixelelectrodes at one of source and drain terminals thereof, tocorresponding one of said row control lines at the other one of thesource and drain terminals, and to corresponding one of said columncontrol lines at a gate terminal thereof; means for supplying a signalto each of said transistors at two signal levels of a reference voltagelevel and an intermediate level; means for addressing said transistorsonce per an addressing period; means for controlling a ratio (a totalperiod of the reference voltage level during the addressing period)/(atotal period of the intermediate level during the addressing period);and means for supplying said reference voltage level at high and low twolevels with said intermediate level between said high and low twolevels, and with said high and low two levels alternated.
 2. The deviceof claim 1 wherein said addressing means can address the row controllines rotationally.
 3. The device of claim 1 further comprising:a firstclock for producing a first period; and a second clock for producing asecond period, wherein one duration of the reference voltage level canbe a natural-number times of said first period and one duration of theintermediate level can be a natural-number times of said first periodand said addressing period can be two or more natural-number times ofsaid first period and said high and low two levels can be alternated atsaid second period.
 4. The device of claim 1 wherein saidelectro-optical modulating layer comprises a liquied crystal selectedfrom the group consisting of a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a nematic liquid crystal and acholesteric liquid crystal.
 5. The device of claim 1 wherein saidelectro-optical modulating layer comprises an organic resin and a liquidcrystal dispersed in said organic resin, said liquid crystal beingselected from the group consisting of a nematic liquid crystal, acholesteric liquid crystal and a smectic liquid crystal.
 6. The deviceof claim 3 wherein said second period is longer than said first period.7. The device of claim 3 further comprising means for synchronizing theproduction of said second period with the production of the first periodwith said second period being two or more natural-number times of saidfirst period.
 8. The device of claim 1 wherein an offset voltage can beapplied to an electrode provided on the other one of said substrates. 9.An electro-optical device comprising:a pair of substrates; anelectro-optical modulating layer provided between said substrates;matrix lines comprising column control lines and row control linesprovided on one of said substrates; a plurality of pixel electrodesprovided on said one of said substrates; p-channel and n-channeltransistor pairs provided on said one of said substrates, each pair ofthe p-channel and n-channel transistors being connected to correspondingone of said pixel electrodes at one of source and drain terminals of thep-channel transistor of said each pair and at one of source and drainterminals of the n-channel transistor of said each pair, to one ofcorresponding pair of the row control lines at the other one of thesource and drain terminals of the p-channel transistor, to the other oneof said corresponding pair of the row control lines at the other one ofthe source and drain terminals of the n-channel transistor, and tocorresponding one of said column control lines at gate terminals of thep-channel and n-channel transistors of said each pair; means forsupplying a signal to each of said p-channel and n-channel transistorpairs at two signal levels of a reference voltage level and anintermediate level; means for addressing said p-channel and n-channeltransistor pairs once per an addressing period; means for controlling aratio (a total period of the reference voltage level during theaddressing period)/(a total period of the intermediate level during theaddressing period); and means for supplying said reference voltage levelat high and low two levels with said intermediate level between saidhigh and low two levels, and with said high and low two levelsalternated.
 10. The device of claim 9 wherein said addressing means canaddress the row control lines rotationally.
 11. The device of claim 9further comprising:a first clock for producing a first period; and asecond clock for producing a second period, wherein one duration of thereference voltage level can be a natural-number times of said firstperiod and one duration of the intermediate level can be anatural-number times of said first period and said addressing period canbe two or more natural-number times of said first period and said highand low two levels can be alternated at said second period.
 12. Thedevice of claim 9 wherein said electro-optical modulating layercomprises a liquid crystal selected from the group consisting of aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, anematic liquid crystal and a cholesteric liquid crystal.
 13. The deviceof claim 9 wherein said electro-optical modulating layer comprises anorganic resin and a liquid crystal dispersed in said organic resin, saidliquid crystal being selected from the group consisting of a nematicliquid crystal, a cholesteric liquid crystal and a smectic liquidcrystal.
 14. The device of claim 9 further comprising red, green andblue color filter films provided on the other one of said substrates.15. The device of claim 11 wherein said second period is longer thansaid first period.
 16. The device of claim 11 further comprising meansfor synchronizing the production of said second period with theproduction of the first period with said second period being two or morenatural-number times of said first period.
 17. The device of claim 9wherein an offset voltage can be applied to an electrode provided on theother one of said substrates.
 18. An electro-optical device comprising:apair of substrates; an electro-optical modulating layer provided betweensaid substrates; a plurality of pixels arranged between said substratesin the form of matrix; means for addressing said pixels once per anaddressing period; means for supplying digital data to said pixels bythe use of two signal levels of a reference voltage level and anintermediate level; means for controlling a ratio (a total period of thereference voltage level during the addressing period)/(a total period ofthe intermediate level during the addressing period); and means forsupplying said reference voltage level at high and low two levels withsaid intermediate level between said high and low two levels, and withsaid high and low two levels alternated.
 19. The device of claim 18wherein said electro-optical modulating layer comprises a liquid crystalselected from the group consisting of a ferroelectric liquid crystal, anantiferroelectric liquid crystal, a nematic liquid crystal and acholesteric liquid crystal.
 20. The device of claim 18 wherein saidelectro-optical modulating layer comprises an organic resin and a liquidcrystal dispersed in said organic resin, said liquid crystal beingselected from the group consisting of a nematic liquid crystal, acholesteric liquid crystal and a smectic liquid crystal.